Classic purifying systems for this type of tool and engines comprise a cartridge filter inserted in the aspiration conduit of the motor.
However, when the air is very polluted, the filter rapidly blocks and normal engine functioning is compromised, or in any case perfect purification is not guaranteed.
To obviate this drawback purifying systems exist which comprise cyclone means destined to separate the heaviest polluting parts of the air, downstream of which the usual cartridge filter is located.
These known systems create the air flow to be filtered with the aid of the engine, which aspirates the environmental air and forces it to cross the cyclone, from the central/axial zone of which the purified air to be sent on exits, possibly with the interposing of a typical cartridge filter, to the aspiration conduit of the engine.
The known systems, though resolving the problem of purification of the aspirated air effectively, are afflicted by a series of drawbacks which limit the use thereof.
A first drawback is that the impurities separated by the cyclone and aspirated by the cooling fan actually foul the cooling fan and the surrounding parts of the engine.
A second drawback lies in the difficulty of properly adjusting the pressure internally of the cyclone, in order to be able to guarantee optimal aspiration of air through the cartridge filter.
In attempts to obviate these drawbacks, solutions have been proposed in which the cyclone is provided with a collecting tank for the detritus which separates from the air, from which a connecting conduit branches and opens into a narrow tract of the motor discharge conduit.
In use, this narrow tract of the discharge conduit behaves as a Venturi tube which creates a depression at the narrow section in which the connecting conduit opens, which thus enables the polluting material to be aspirated from the collecting tank.
In this way, the polluting material is discharged externally through the exhaust pipe, together with the exhaust gases, without fouling the motor or the tool associated thereto.
A system of this type is illustrated, for example, in European Patent No. EP0558091.
This type of purifying system is, however, also not free of drawbacks. In particular, the Venturi tube constitutes a choke in the discharge conduit, which obstructs the outflow of the exhaust gases, causing significant load loss.
To obviate this load loss, a significant part of the energy produced by the motor has to be expended in increasing the exhaust gas pressure, and cannot therefore be effectively exploited to work the tool to which the motor is associated, thus reducing its performance and increasing the overall consumption of fuel.
The presence of a choke on the discharge conduit further lowers the overall performance of the motor.
The choke modifies the discharge system which, as is known, is a determinant factor for obtaining good performance in two-stroke engines of simple construction, such as those used for this type of tool.